In mobile systems, signal fading over the radio path interferes with reliable transmission. The problem is aggravated in new systems where rather high data transfer rates are transmitted, in addition to speech, together with new services, such as data transmission, which require a very good bit error ratio.
A possible solution to the problem is the use of transmit diversity. This means that the base station transmits a signal to a user equipment by means of two or more different antennas. Therefore the multipath signal components which have propagated via different channels will not likely be disturbed by simultaneous fading.
In selective transmit diversity (STD), the base station transmits a signal to a user equipment by using at least two different antenna candidates. The user equipment measures the quality of the signal transmitted by each antenna candidate and selects the antenna that provides the best quality. The user equipment signals the identification data of the selected antenna to the base station, whereafter the network part of the mobile system guides the transmissions to the user equipment via the selected antenna. This kind of signalling method forms closed loop control. A problem with this method is that the user equipment must be able to reliably signal the identification data of the selected antenna to the network part. STD is described in Transmit Diversity by Antenna Selection in CDMA Downlink by Ari Hottinen and Risto Wichman (IEEE Fifth International Symposium on Spread Spectrum Techniques & Applications. IEEE ISSSTA '98 Proceedings. Sep. 2–4, 1998, Sun City, South Africa), which is incorporated herein by reference.
Another manner of implementing transmit diversity is the use of Space-Time Transmit Diversity (STTD). The operating principle of STTD differs from STD in that in STTD a signal is transmitted continuously to a user equipment by means of at least two different antennas. The signals that are transmitted via separate antennas are different. There are two manners of implementing the difference: space-time trellis codes and space-time block codes.
Space-time trellis codes are described in WO 97/41670, which is incorporated herein by reference. They provide both coding and diversity gain. The codes are formed by means of a trellis diagram, which describes with two symbols each possible state and branches to other states. When the initial state of the trellis is known, the bits to be coded can be indicated in the trellis diagram by means of symbols denoting transfers between different levels. The obtained symbols are thereafter distributed for transmission via different antennas.
In space-time block codes the bits to be coded are divided for example into two-bit sequences, which are formed into symbols to be transmitted, such that the symbol to be transmitted via the first antenna consists of the first bit and the complex conjugate of the second bit, and the symbol to be transmitted via the second antenna consists of the second bit and the complex conjugate of the first bit. The formation of space-time block codes is described in Space-Time Coding for High Data Rate Wireless Communications by A. R. Calderbank, Hamid Jafakhani, Ayman Naguib, Nambi Seshadri and Vahid Tarokh (Fifth Workshop on Smart Antennas in Wireless Mobile Communications. Jul. 23–24, 1998, Stanford University), which is incorporated herein by reference.
In STTD, the transmit power of transmit antennas is constant or it can be controlled by means of closed loop control, wherein a user equipment measures the quality of the signals it has received, and based thereon, the network part adjusts the absolute transmit power of the signal it transmits via the antennas, such that the ratio of the transmit powers is always the same over each transmit antenna path utilizing transmit diversity. However, the arrangement can cause an unnecessarily great deal of interference to other users in the mobile system. This method also has the problem of reliability of signalling, in other words the user equipment must be able to reliably signal power control data to the network part.